Radiotherapy Flashcards
What is the definition of radiotherapy?
the treatment of diseases (most commonly, cancer) using ionising radiation
With what 4 different intents can radiotherapy be given with?
Radical / curative:
when radiotherapy is given as a way to cure cancer (often with chemotherapy)
Adjuvant:
when radiotherapy is given in addition to curative surgery to reduce risk of local recurrence
Palliative:
to help symptom control at the end of life
Neo-adjuvant:
radiotherapy given prior to a definitive procedure to shrink the cancer beforehand
When and where can radiotherapy be received?
- available in tertiary hospitals only
- given as an out-patient on weekdays
What are the 3 defining characterstics of radiotherapy?
- it is invisible
- it is silent
- it is pain free to receive
What are the 2 different ways of delivering radiotherapy?
What equipment is involved?
it can be delivered using photons or electrons
these are generated and delivered by a linear accelerator
most radiotherapy in the UK uses photon radiotherapy
How does photon radiotherapy work?
- photons are high energy XRs that penetrate deep into the body tissue
- within the tissue, they produce secondary electrons
- these electrons cause DNA damage to both cancer cells and normal cells
Why does photon radiotherapy work even though normal cells are damaged?
- normal cells can repair radiotherapy induced damage (up to a point)
- tumour cells have defective DNA repair mechanisms and cannot repair from radiation-induced damage
When might electron radiotherapy be used opposed to photons?
- electrons are used to deliver treatment to relatively superficial lesions (e.g. skin mets)
- photons will penetrate deep into the body tissue but spare the overlying skin
Under what rare circumstances is proton radiotherapy used?
Why is it used in this situation opposed to photons?
- protons are used in young patients who often have brain tumours
- a proton will hit the area of concern and not penetrate any further (no exit dose)
- whereas a photon will hit the area of concern and continue travelling through the tissue
want to deliver as little radiation to developing brain tissue
Why is radiotherapy effective at killing cancer cells?
- cancer cells have defective DNA repair mechanisms and cannot repair from radiotherapy-induced damage
- they undergo apoptosis or mitotic cell death
How can radiotherapy cause secondary cancers if normal cells can repair themselves?
- normal cells can repair the DNA damage that occurs as a result of radiotherapy
- if radiotherapy is constantly given to a healthy cell, eventually it will be damaged to a point where it can no longer repair itself
- this results in late side effects and secondary cancers
What is the unit used to measure radiotherapy?
the absorbed dose of radiation is expressed as the unit “Gray” (Gy)
How are the doses of radiotherapy delivered?
- RT is delivered as a series of small doses called fractions
- the number of fractions and the dose given in each fraction (Gy) depends on the treatment intent
What is the typical RT regime for radical / curative RT?
- radical RT requires larger doses of radiotherapy overall
- the total dose is divided into multiple small fractions
each fraction is often around 2 Gray
e.g. 70Gy may be delivered in 35 fractions over 7 weeks
there are more frequent smaller doses compared to fewer larger ones
What is the typical RT regime for RT with palliative intent and why?
- as this treatment is not curative, the aim is for it to be straightforward with minimal time in hospital
- not as concerned about late side effects in this group
- it is delivered in a smaller number of fractions and to a lower total dose
each dose tends to be higher (> 2 Gy) compared to curative RT
e.g. often 8Gy given in 1 fraction or 30Gy in 10 fractions
What are the 5 pre-treatment stages that must occur before someone has RT?
- imaging + diagnosis
- patient consultation
- consent
- immobilisation
- planning CT scan
What are the 5 stages involved in the treatment planning phase of RT?
- delineation of the tumour
- addition of margins
- delineation of normal tissue
- radiotherapy treatment planning
- planning review + prescription
What are the 4 stages involved in the treatment delivery stage of RT?
- attendance for daily treatments
- treatment delivery + verification
- clinical review during treatment
- long-term follow up
Before a patient signs a consent form for radiotherapy, what key areas must be discussed with them?
- what you are going to do
- why you are going to do it
- intended benefits
- possible side effects
- alternative options
How is immobilisation acheived for RT?
- patients with H&N cancer will have a mask made that they must wear for every treatment
- this ensures the desired area is being targeted and it is reproducible
Other than masks, what other types of immobilisation may be used?
vacuum bags are used to immobilise the torso / legs
What are the 2 main differences between a CT simulator and normal CT scan?
hard, flat bed:
this helps to keep the patient in the same fixed position for every treatment
reference marks:
there are marks along the side of the bed that the patient is lined up to for reference
How are patients able to be lined up with the reference marks on the CT simulator?
- patients are given 3 tattoos - one in the centre and on either side
- the tattoos are lined up on the couch in reference to the CT scans
What are the 3 key volumes that are used to design where the RT is targeted?
- gross tumour volume (GTV)
- clinical target volume (CTV)
- planning target volume (PTV)
What is the clinical target volume?
- tumours often** do not stop at the margins** that are visible (GTV)
- there is often microscopic disease outside of the GTV area
- a margin is put on to take into account this microscopic spread - this is the CTV
What is the planning target volume (PTV) and why is it needed?
- tumours can move about and are never in the same place day-to-day despite immobilisation
- a margin needs to be introduced to take into account potential variations in tumour position (PTV)
e.g. if the bladder is full, then the prostate tends to be lower down
What is represented by the blue, green and red lines?
red - GTV:
this is a margin drawn around the tumour that can be seen
green - CTV:
this is a margin to account for microscopic spread
blue - PTV:
this is a margin to account for tumour movement / day-to-day variation
How is the CT planning of palliative radiotherapy different to radical?
- the aim is for a relatively simple design with low toxicity
e.g. pelvic met outlined in red
there are 2 beams (from front / back) that meet in the middle where the tumour is
What is meant by the “glancing pair” arrangement of RT beams and why is it used?
- used for adjuvant RT to the breast following wide local excision
- one beam comes from the left and one from the right to “glance” across the chest
- this encompasses all of the breast, but minimises the dose to the lungs / heart
using one beam from the front would deliver a large exit dose to the lungs / heart
How does the linear accelerator work?
- laser lights are used to align the patient with reference to their tattoos
- RT comes out of the head of the LA
- the grey circle is the gantry that allows RT to be delivered through 360 degress
Why are their leaves of tungsten present in the head of the LA?
- the tungsten leaves are used to shape the RT beams
- they move in and out to create an aperture through which the RT can pass through
How does radiotherapy dermatitis tend to appear at the start and end of treatment?
- around a week into treatment, the skin appears mildly red
- after around 7 weeks of radical radiotherapy, the skin peels off and is very red
- this continues for 2 weeks following treatment and then it starts to heal
- even after healing, the skin may remain slightly pigmented due to irreversible fibrosis of the skin
Why do a lot of patients who have H&N RT lose weight during their treatment?
oral mucositis
- the inside of the mouth becomes red, swollen and begins to peel
- it becomes very difficult to eat as a result
